(1) Field of the Invention
The present invention relates to a manufacturing method for an arc tube in which a glass tube is turned to form a turning part therein and is spirally wound from the turning part to at least one end part in a first direction to form a bent glass tube, and in which a phosphor layer is formed on an inner surface of the bent glass tube.
(2) Description of the Related Art
In this age of energy conservation, compact self-ballasted fluorescent lamps that are highly lamp efficient and have a long life are being proposed as light sources in place of common incandescent lamps. As such a compact self-ballasted fluorescent lamp, inventors are investigating the use of arc tubes formed in a double spiral by turning a glass tube in substantially a center thereof, and winding both ends thereof on a predetermined axis (hereinafter called a “spiral axis”) (see Japanese Laid-Open Patent Nos. 8-339780 and 9-17378).
The reason for such investigations into the use of arc tubes with a double spiral shape is that this kind of arc tube has a longer discharge path than a so-called three-U shape that consists of three U-shaped glass tubes joined together, even when housed in the same capacity.
The glass tube that has been formed into a double spiral shape and that composes an arc tube is called “double spiral glass tube”. A phosphor layer is formed on the inner surface of the double spiral glass tube. FIG. 1 shows a method for forming the phosphor layer on the inner surface of a double spiral glass tube 509. The following describes a conventional phosphor layer formation method with use of FIG. 1.
First, the double spiral glass tube 509 is positioned so that one end faces upward, as shown in FIG. 1(a), and a suspension for forming the phosphor layer is injected into the double spiral glass tube 509 through an end part 591 thereof. Here, the “end parts” of the spiral tube denote the end parts of the glass tube that composes the spiral tube.
When the suspension has been injected, the double spiral glass tube 509 is stood so that the end part 591 is at the top, and the injected suspension flows downward to the end of the double spiral glass tube 509 at which the turning part 592 is formed. Here, the double spiral glass tube 509 is shaken gently so that the suspension coats the entire inside surface while also reaching the turning part 592 quickly.
Next, when the suspension has flowed to the end at which the turning part 592 is formed, the double spiral glass tube 509 is turned upside down as shown in FIG. 1(c), and the suspension drains out due to its own weight. When the suspension has substantially finished draining out, the double spiral glass tube 509 is positioned so that the other end faces upward, suspension is injected through the other end part, and once again the double spiral glass tube 509 is tuned upside down and the suspension is drained (FIGS. 1(a), (b), and (c)).
After having the injected suspension drain from both end parts 591 in this way, the double spiral glass tube 509 is maintained in the standing state such that the turning part 592 is upward, and is subject to an atmosphere of 100° C. while warm air is blown into the double spiral glass tube 509 through the end part 591 as shown in FIG. 1(d). This preliminary drying is performed until the suspension coating the inner surface of the double spiral glass tube 509 loses fluidity. Finally, the suspension is dried completely by placing the double spiral glass tube 509 in a drying oven for eight minutes at 45° C. as shown in FIG. 1(e). The described processes result in a phosphor layer being formed on the inner surface of the double spiral glass tube 509.
In this conventional method, the suspension in the double spiral glass tube 509 is dried after being drained, while the double spiral glass tube 509 is standing. For this reason, the suspension in the spiraling parts of the double spiral glass tube 509 flows downward from the top side of the surface in a cross section. Accordingly, in the cross section of the double spiral glass tube 509, the phosphor layer is formed thinly on the surface on the top side and thickly on the surface on the bottom side (hereinafter, parts where the phosphor layer is formed thickly are referred to as “thickly-formed parts”.
FIG. 2 shows a lamp that uses an arc tube manufactured according to the above-described conventional manufacturing method, during illumination. Since visible light excited in the phosphor layer is unable to pass through the thickly-formed parts in the lamp that uses the conventional arc tube, the thickly-formed parts are darker than other parts, and appear as bands of shadow shown by hatching in FIG. 2. This is not a problem if a globe is provided to cover the arc tube because the arc tube is not visible from outside the glove. However, such dark parts are undesirable in terms of design in a lamp that does not have a globe because they are visible.